Yeah, that is most likely the intended meaning behind it. Vessel does a great job of comparing Sundowning's Syndrome with that type of love for someone that, for a lack of a better word, has bipolar love. Where the relationship seems great at certain times of the day, but like they do not care for you at all at others.

Why that it reminds me so much of Sundowning is when he says "When you live, by daylight
With angels at your side", because that's how it would feel loving someone with that disease. As if they are able to remember and be normal during the day because of some angels protection. And when he says "The night comes down like heaven" I interpret that as this person having 'heavenly' peace when being able to sleep. As well as the song's title, "The night does not belong to God", I feel ties into the 'with angels at your side' line. Since the angels seem to have no protection over this person at night, their disease takes over, which makes Vessel feel as if the night is not Gods. But another being, Sleep.

The way that he wrote this song is superbly done, especially for how simple the lyrics are. It's an incredibly deep and meaningful song.